The SFO said in a statement Tuesday: “While there were reasonable grounds to suspect the commission of offences involving serious or complex fraud, a detailed review of the available evidence led us to the conclusion that the alleged conduct, even if proven and taken at its highest, would not meet the evidential test required to mount a prosecution for an offence contrary to English law.

“It has further been concluded that this evidential position could not be remedied by continuing the investigation.”

The 2014 probe was launched after the SFO was handed material by the Financial Conduct Authority.

This probe was launched after the United States' Department of Justice teams had started investigating whether currency traders manipulated markets to enhance their positions.

The SFO said today that it is continuing to liase with the US Department of Justice as part of its ongoing investigation.

Alison McHaffie, a regulatory partner with law firm CMS, said this would be "embarrassing" for the SFO.

She added: "However it does show the difficult job SFO has in demonstrating criminal activity by individuals for this type of type of market misconduct and without a change in the law on corporate criminal responsibility.

"This means it is always easier to impose regulatory fines against the firms themselves rather than criminal prosecutions.”